1. Field of the Invention
This invention relates to an optical communication apparatus such as a transmitter, a receiver and a transmitting-receiving device, and especially a surface-mounted module.
2. Description of Related Art
Modules in practical use for optical transmission at present have a three-dimensional structure with a laser diode (hereinafter referred to as xe2x80x9cLDxe2x80x9d), a photo diode (hereinafter referred to as xe2x80x9cPDxe2x80x9d), a lens, and a ferule for supporting an optical fiber end.
FIG. 1 is a cross sectional view of an LD module in practical use. An LD 3 is fixed on a side of a pole 2 equipped near the center of a metallic stem 1, and a monitoring PD 4 is fixed on the center of the metallic stem 1. A cylindrical cap having an opening 5 and a cylindrical lens holder 6 are equipped on a metallic stem 1. A lens 7 is fixed at the opening of the lens holder 6. A conical ferule holder 8 is fixed on the lens holder 6. A ferule 10 for holding a tip of an optical fiber 9 is inserted into the top of the ferule holder 8. Light emitted from the LD 3, perpendicular to the stem surface, is focussed by the lens to enter the optical fiber 9 in the ferule 10.
Since this module is mounted in a metallic package and has many adjusting parts, it is reaching a limit in terms of cost reduction and minimization of the module because of its structure.
Therefore, a surface-mounted optical transmission module was proposed in order to reduce and minimize cost. In the module, a V groove for putting an optical fiber therein was provided on a Silicon (hereinafter referred to as xe2x80x9cSixe2x80x9d) substrate, and an optical device such as an LD or a PD was mounted on the Si substrate without any adjustments. The Si substrate used in the module differed from Si wafer material used in semiconductor industries.
A proposed configuration of the module is:
a rectangular silicon plate provided as a substrate,
an optical fiber end or a ferule having an optical fiber inserted thereinto, which is fixed with adhesives or the like on the substrate, and
an optical device such as an LD or a PD provided on the substrate adjacent to the end of the fiber and aligned on the light axis prolongation.
In this module, an Si substrate is used merely as a base plate, and the conductivity of the Si substrate is unnecessary. The conductivity is rather troublesome. A high-resistivity silicon single crystal is used as an Si substrate. After forming an insulation layer, a metallized pattern is formed on the insulation layer. The material of the insulation layer is SiO2 or SiN.
This module is two-dimensional, and the surface of the substrate is parallel to the light direction. An LD or a PD is fixed after being adjusted by the mark provided on the substrate. Optics systems for collecting light such as a lens are omitted because the optical distance between an optical device and an optical fiber end is very short. It was said that the optical fiber and the optical device were securely connected because of adopting a photolithography technology, and this module reduced cost because of the small number of parts. (Reference 1, xe2x80x9cPassive Alignment Technique for LD Module using Si Platformxe2x80x9d, by Mr. Seimi Sasaki et al, EMD 95-27, CPM 95-53, OPE 95-50 (1995-08), Technical Report of The Institute of Electronics, Information and Communication Engineers)
Another type of surface-mounted device combined with a plural number of fibers, optical wave-guides and transmitting and/or receiving optical devices was also proposed. For example, xe2x80x9cResearch on the mounting method of optical devices for SFF Optical Transceiverxe2x80x9d, Preprint of the congress, C-3-28 p133, 1999 by Ryu-ta Takahashi, Murakami Kazuya, Sunaga Yoshinori, Tokoro Takehiko and Kobayashi Masahiko in 1999 Congress of the Electronics Society of Electronics and Information Society (Reference 2).
However, there was a problem from the standpoint of electrical crosstalk (hereinafter electrical crosstalk referred to as xe2x80x9ccrosstalkxe2x80x9d). Crosstalk is phenomena in which electric signals for an LD driver go around a substrate and an insulation layer to a PD, when the LD and the PD are placed on the substrate and they transmit and receive signals simultaneously. The lowest crosstalk is the most favorable. To reduce crosstalk, an Si substrate having higher resistivity such as 10 xcexa9cm to 100 xcexa9cm was normally used. Furthermore a module using an Si substrate having 1 kxcexa9cm was published. (Reference 3, xe2x80x9cA Low-crosstalk Optical Module Design on PLC Platform for Realizing LD/PD Full-duplex Operation in ATM Systemsxe2x80x9d, by Hideki Kimura et al., ECOC 98, Sep. 20-24, 1998, Madrid, Spain) However, it was insufficient for reducing crosstalk.
Another method to reduce crosstalk was proposed by T. Ikeuchi, Y. Tochio, K. Mori, T. Yamamoto, H. Rokugawa, A. Abe, S. Yamada, K. Shimizu and M. Kawai in ECOC""99, Sep. 26-30, 1999, Nice, France. (Reference 4, xe2x80x9cHigh Sensitivity ATMPON PLC Transceiver (ONU) by Unique Crosstalk Reduction Methodxe2x80x9d, Preprint of the congress, p. I-330)
In this reference the authors reported they achieved a removal of crosstalk by passing output signals of the preamplifier (AMP) to a low path filter (LPF) at 155 MHz. However, this method of adding parts such as LPF is not the best solution in terms of cost, which is the most important factor in this Planar Lightwave Circuit (PLC) technology field.
Moreover, the crosstalk reduction method was incomplete. This method is able to prevent high frequency crosstalk, however, crosstalk signals leaking from LD not only have high frequency noise but also low frequency LD noise.
The present invention is related to a module characterized in that an electrical potential level of a Si substrate is close to the conductor level and the Si substrate is firmly connected to the ground potential level.
It is impossible for a high-resistivity Si substrate to be a good ground, even if the Si substrate is partly grounded. By the present invention, a low resistivity Si substrate becomes an effective ground even if only a part of the Si substrate is grounded.
Crosstalk between an LD and a PD is prevented because there is a ground level surface nearest under the bottoms of electrode patterns for an LD and a PD. A transmission device, which is strong against outer noise and internal crosstalk, can be achieved by grounding the Si substrate. Moreover, the crosstalk is completely removed by providing patterned ground terminals (hereinafter patterned ground terminal(s) referred to as xe2x80x9cground terminal(s)xe2x80x9d) among electrodes on the surface of the Si substrate.
Additionally, the Si substrate of the present invention is advantageous in the view of obtainability and cost reduction. A high-resistivity Si single crystal is special. The crystal has a small market and there is no mass production of the crystal. Therefore, it is difficult to obtain, and it is expensive. On the other hand, a low resistivity Si substrate applied to the present invention is made of an ordinary n-type Si single crystal most frequently used in the Si semiconductor industry. Therefore, the low resistivity Si substrate is less expensive, and consequently a device cost can be reduced.
Thus, the resistivity of the Si substrate in the present invention is positively lowered, and the substrate is used as a ground plate. Ground terminals are provided among electrode patterns for optical devices. More preferably a copper contact adhered to the bottom surface of the Si substrate is connected to the ground terminals.
Features that may be present in some implementations of the present invention are summarized as follows:
1. A low-resistivity Si-single crystal is used as an Si substrate.
2. Electrical potential of the Si substrate is a ground potential level.
3. A metallic contact such as a copper contact is adhered to a bottom surface of the Si-substrate.
4. Ground terminals are provided on the Si substrate to prevent electrical connection between each of the electrodes for optical devices.
It was presumed in the past that the higher resistivity of an Si substrate the better. To the contrary, however, the inventors discovered, as described above, that the lower the resistivity of an Si substrate the better. The high-resistivity Si substrate is expensive because of difficulties of production and small markets. On the other hand, the low-resistivity Si substrate can achieve a cost reduction because of easily obtainable mass-manufactured products in the semi-conductor industry.
Moreover, the present invention positively uses an Si substrate as a ground with introducing new concepts of a comb-tooth-shaped grounding structure placed among the electrode patterns for optical devices. The internal crosstalk between an LD and a PD during transmitting and receiving signals can be restrained, and also the PD can be protected from an external noise because of the combination of the low-resistivity Si substrate and the metallized grounding terminals surrounding the electrode patterns. As a resultant a low cost and high performance optical transmission apparatus can be achieved.